Monatomic boron ion source and method

ABSTRACT

Monotomic boron ions for ion implantation are supplied from decaborane vapor. The vapor is fed to a plasma chamber and a plasma produced in the chamber with sufficient energy density to disassociate the decaborane molecules to produce monatomic boron ions in the plasma.

FIELD OF THE INVENTION

The invention relates to a source of, and a method of providing,monatomic boron ions for ion implantation.

BACKGROUND OF THE INVENTION

Boron is a well known dopant used for modifying the conductivity ofsemiconductor materials in the manufacture of integrated electroniccircuits. Monatomic boron ions (B⁺) are commonly implanted into siliconfor this purpose. A typical ion source used for generating an ion beamcontaining monatomic boron ions uses BF₃ gas as the feed gas to theusual plasma chamber of the ion source. In the ion source, the BF₃ gasis disassociated in the plasma to form B⁺ ions, often as well as BF⁺ andBF₂ ⁺. The ion beam extracted from the ion source is passed through amass analyser to select the B⁺ ions for onward transmission forimplanting in the semiconductor wafer target.

It is also known to use decaborane (B₁₀H₁₄) as a feed stock for an ionsource in ion implantation. Decaborane is used to produce ions eachcomprising up to 10 boron atoms. Such B_(x)H_(y) ⁺ ions can be used toimplant boron atoms at relatively low energies.

Decaborane Ion Implantation by Perel et al, IIT 2000, pp. 304 to 307,discloses the spectrum of ion masses which may be generated from asuitably controlled ion source employing decaborane as feed stock. Ionshaving masses corresponding to the presence of 10 boron atoms areselected in a mass analyser for implantation.

U.S. Pat. No. 6,288,403 discloses an ion source adapted for thepreferential production of decaborane ions, particularly for low energyimplantation.

SUMMARY OF THE INVENTION

The present invention provides a method of providing monatomic boronions for ion implantation, comprising supplying decaborane vapour into aplasma chamber, and generating a plasma in said plasma chamber having asufficient energy density to disassocite decaborane molecules to producemonatomic boron ions in the plasma.

In the present invention, decaborane vapour is fed to the plasma chamberin order to provide a supply of boron atoms in the plasma to enhance thecurrent of monatomic boron ions which can be extracted from the source.At least initially, a different plasma supporting gas may be supplied tothe plasma chamber of the ion source, such as BF₃ or Ar. The plasmasupporting gas allows a stable plasma to be established initially in theplasma chamber. When the plasma chamber is hot enough, the flow ofsupporting gas can be backed off in favour of the decaborane vapour. Arelatively high energy density plasma is maintained within the plasmachamber and the decaborane vapour provided in the plasma chamber is thendissociated in the plasma to provide monatomic boron for inclusion inthe extracted ion beam.

The invention also provides a source of monatomic boron ions for an ionimplanter, comprising a plasma chamber, decaborane vapour supply, asupply of a plasma supporting gas, other than decaborane vapour, anenergy supply to said plasma chamber to form a plasma therein having anenergy density sufficient to disassociate decaborane molecules toproduce monatomic boron, and a controller to control said decaboranevapour supply and said supporting gas supply to provide a simultaneoussupply to the plasma chamber of decaborane vapour and said supportinggas.

It may be convenient to ensure that a feed conduit of the decaboranevapour supply to the plasma chamber is cooled so that the decaboranevapour is kept below 300° C. before entering the plasma chamber. Thishelps prevent dissociation of the decaborane before entering the plasmachamber and reduces deposition of the dissociation products in the feedconduit.

Normally, the ion source is used in combination with a mass selector setup to form a beam of monatomic boron ions for transmission to thesubstrate to be implanted.

BRIEF DESCRIPTION OF THE DRAWING

An example of the invention will now be described with reference to theaccompanying drawing which is a schematic diagram of an ion sourceembodying the invention and in combination with a mass selector.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawing, an ion source has a plasma chamber 10 in whichfeed gas is ionised to form a plasma 11 containing ions of an atomicspecies to be implanted in a substrate (not shown). Ions are extractedfrom the plasma chamber 10 through an extraction aperture 12, by meansof an extraction electric field formed by suitably biased extractionelectrodes 13, 14. The extracted ions are accelerated by electrodes 13and 14 to form an ion beam 15 which is directed into a mass analyser 16.The mass analyser may, in accordance with known practice, be a magneticsector analyser, in which ions, entering the analyser 16 with theselected momentum, pass through the analyser in a path with a curvaturesuch that the selected ions pass through a mass selection slit 17 at theexit of the analyser, to form a beam of mass selected ions 18, foronward transmission to a process station of an ion implanter which isnot shown in this drawing.

The plasma chamber 10 may be a DC arc type plasma chamber, in whichenergy is delivered to maintain the plasma in the chamber, from an arcsupply 19. The arc chamber arrangement may, for example, be the wellknown Bernas-type, in which thermionic electrons emitted by a cathode inthe chamber are confined to an axial region of the arc chamber by meansof an applied magnetic field.

Feed gas is supplied to the arc chamber 10 to maintain a desired partialpressure within the arc chamber sufficient to support plasma 11. Inknown ion sources, a beam of boron ions is produced by feeding BF₃ gasto the arc chamber. Within the arc chamber the arc supply 19 iscontrolled to generate a plasma of sufficient energy density todisassociate the BF₃ molecules and to form within the plasma ions of B⁺,as well as BF⁺, and possibly BF₂ ⁺. If it is desired that beam 18, fortransmission to the implant process chamber, is a beam of B⁺ ions, themass analyser 16 is set to reject other ions generated in the arcchamber and extracted in the initial beam 15. Clearly, in order tomaximise the B⁺ current in beam 18 from the mass selector, the arcchamber 10 is operated to maximise the proportion of B⁺ ions in theplasma 11.

In accordance with standard practice, the BF₃ feed gas supply to arcchamber 10 comprises a gas bottle 20 connected via a control valve 21and a feed conduit 22, into the interior of the plasma chamber 10. Therate of supply of BF₃ gas to the arc chamber 10 is controlled by thecontrol valve 21 under the supervision of feed gas supply controller 23.The feed gas supply controller 23 itself receives supervisory controldata from an implanter control system 24, which receives various senseparameter data from the implanter system over a generalised input line25, and supplies control parameter data to control the overallfunctioning of the implanter, over generalised output control lines 26,27, as well as control line 28 to the feed controller 23.

In addition to the BF₃ gas supply illustrated in the FIGURE, thedescribed example of the invention includes a decaborane vapour supply,indicated generally at 30. The decaborane vapour supply 30 comprises anoven 31 fitted with a heater 32, the heat output of which is controlledby the feed controller 23 in response to temperature feedback, fromtemperature sensor 33.

The oven 31 contains a mass of decaborane powder 34 which is heated to atemperature at which the decaborane powder sublimes to provide a desireddecaborane vapour pressure. Decaborane vapour is fed along conduit 35from the oven 31 to supply the decaborane vapour to the interior of thearc chamber 10.

A vapour supply control valve, not shown in the FIGURE, may also beincluded in the vapour conduit 35, to control the rate of flow of vapourfrom the oven 31 into the arc chamber 10. The control valve is thensubject also to control by the feed controller 23.

Decaborane powder has a vapour pressure of the order of 0.1 Torr at roomtemperature, and produces a substantial vapour pressure at temperaturesabove 100° C. However, at temperatures much above 300° C., thedecaborane molecule tends to dissociate. Within the arc chamber 10, thewalls of the arc chamber may be at temperatures of between 500° C. andas much as 1000° C. Furthermore, the arc supply 19 is such that theplasma 11 has an energy intensity which would tend to dissociatesubstantially all decaborane molecules within the plasma region. Theresulting increased number of monatomic boron atoms substantially booststhe monatomic boron ion concentration within the plasma 11, permittingthe extraction of relatively higher monatomic boron ion currents fromthe plasma chamber 10, resulting in an increase in the B⁺ current inmass selected beam 18.

As mentioned above, the decaborane molecule is unstable at temperaturesabove about 300° C. At such higher temperatures, the moleculedissociates and the resulting fragment molecules, including monatomicboron, have a much lower vapour pressure at those temperatures andtherefore tend to deposit out as solid boron. In order to preventdecaborane vapour from dissociating and depositing out within theconduit 35, the conduit 35 is cooled, especially at its connection withthe plasma chamber 10, by means of a cooling jacket 36. The coolant maybe water. The cooling jacket 36 is controlled to ensure that the conduit35 is held at a sufficient temperature to maintain the required vapourpressure of decaborane, but below the temperature (about 300° C.) atwhich the decaborane tends to dissociate. In this way, the decaboranevapour can be fed directly into the interior of the plasma chamber 10without dissociating, thereby ensuring a proper supply of the decaboraneinto the plasma chamber and avoiding deposition of decaborane productswithin the conduit 35.

Inside the plasma chamber 10, the decaborane vapour quickly dissociatesto enrich the B⁺ content of the plasma 11.

In operating the plasma chamber 10 with decaborane vapour feed asdescribed above, the arc within the chamber 10 is first formed using BF₃feed alone at a predetermined rate of supply. Then decaborane vapour isadded to the feed to produce the desired B⁺ enrichment of the plasma.The rate of supply of BF₃ gas may then be reduced. In order to maintaina stable plasma of substantial energy density within the chamber 10,some BF₃ gas may be supplied continuously simultaneously with thedecaborane vapour. However, in some arrangements it may be possible toreduce the second rate of BF₃ supply to zero and to run the plasma ondecaborane vapour alone.

A primary function of the BF₃ feed gas is to facilitate starting theplasma and then, when supplied simultaneously with decaborane vapour, tomaintain plasma stability. This functionality could be achieved byalternate supporting gases compatible with the desired process. Forexample the decaborane vapour could be run simultaneously with argongas, where the argon provides plasma stability and the decaborane vapourenriches the plasma with B⁺ ions.

The feed gas supply controller 23 may be arranged to optimise the ratioof supply of the decaborane vapour and the plasma supporting gas such asBF₃, so as to maximise the B⁺ current in the extracted beam, whilecontrolling or limiting the deposition of boron in the plasma chamberand ensuring a stable plasma.

In the described example, the plasma chamber 10 is constituted by an arcchamber, and the plasma generating energy is derived from an arc supply19. Instead, the energy required to create the plasma within the plasmachamber can be derived from other sources, including radio frequency ormicrowave sources. Any suitable arrangement may be employed forextracting ions from the plasma chamber including a so-called tetrodesystem with four electrodes including the front face of the plasmachamber with the extraction aperture.

Also, although a single aperture 12 for extraction of the plasma to formthe ion beam 15 is illustrated in the drawing, multiple apertures may beprovided, for example for enhancing the total beam current drawn fromthe chamber. Further, the disclosed magnetic sector analyser 16 is justone form of mass analyser which may be used with the described system.

1. A method of providing monatomic boron ions for ion implantation,comprising supplying decaborane vapour into a plasma chamber, andgenerating a plasma in said plasma chamber having a sufficient energydensity to disassociate decaborane molecules to produce monatomic boronions in the plasma, wherein a plasma supporting gas, different fromdecaborane vapour, is supplied at least initially when the plasma isfirst established in the plasma chamber and the plasma supporting gassupply is maintained simultaneously with the supply of decaboranevapour, the rate of simultaneous supply of the supporting gas beingreduced when the plasma chamber reaches a desired temperature.
 2. Amethod as claimed in claim 1, wherein the supporting gas is BF₃.
 3. Amethod as claimed in claim 1, wherein the supporting gas is Ar.
 4. Amethod as claimed in claim 1, wherein the decaborane vapour is keptbelow 300° C. before entering the plasma chamber.
 5. A method as claimedin claim 1, wherein ions are extracted from the plasma chamber and massselected to form a beam of monatomic boron ions.
 6. A method ofproviding monatomic boron ions for ion implantation, comprisingsupplying decaborane vapour into a plasma chamber, and generating aplasma in said plasma chamber having a sufficient energy density todissociate decaborane molecules to produce monatomic boron ions in theplasma, wherein BF₃ is supplied at least initially as a plasmasupporting gas when the plasma is first established in the plasmachamber.
 7. A method as claimed in claim 6, wherein the decaboranevapour is kept below 300° C. before entering the plasma chamber.
 8. Amethod as claimed in claim 6, wherein ions are extracted from the plasmachamber and mass selected to form a beam of monatomic boron ions.
 9. Amethod of providing monatomic boron ions for ion implantation,comprising supplying decaborane vapour into a plasma chamber, generatinga plasma in said plasma chamber having a sufficient energy density todissociate decaborane molecules to produce monatomic boron ions in theplasma, extracting ions from the plasma chamber using biased electrodesto form a beam of extracted ions, directing the beam of extracted ionsinto a mass analyzer, controlling the mass analyzer to selectsubstantially only monatomic boron ions from the beam of extracted ionsto form a continuing beam of substantially only monatomic boron ions,and transmitting the continuing beam of substantially only monatomicboron ions to a substrate to be implanted therein.
 10. A method asclaimed in claim 9, wherein the plasma chamber is an arc type plasmachamber and energy is delivered to maintain the plasma in the chamberfrom an arc supply.
 11. A method as claimed in claim 9, wherein energyto generate the plasma in the plasma chamber is derived from one ofradio frequency and microwave sources.